ESTIMATING THE SPECIFlC GRAVITY 

OF PLYWOOD 

June 1944 

This Report is One of a Series 

Issued In Cooperation with the 

ARMY-NAVY-CIVIL COMMITTEE 

on 

AIRCRAFT DESIGN CRITERIA 

Under the Supervision of the 

AERONAUTICAL BOARD 

No. 1589 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison, Wisconsin 

ln Cooperation with the University of Wisconsin 
ESTIMATING THE SPECIFIC GRAVITY OF PLYWOOD1 

By 

BENSON H. PAUL, Senior Silviculturist 

and 

JOHN P. LIMBACH, Assistant Forest Products Technologist 

When veneer is manufactured into plywood the resulting product has a 

greater specific gravity than the veneer because of the cummulative influence 

of a number of factors. These factors include the pressing conditions, the 

weight of bonding medium applied to a unit area of veneer before pressing 

into plywood, the number of plies in a given thickness of plywood, the difference in the volumetric shrinkage of plywood and veneer, and the average 

specific gravity, weighted as to thickness, of the material comprising the 

plies. 

It is possible to estimate what the increase in specific gravity will 

be, as follows: 

1. Using the specific gravity of the veneer plies obtained from the 

method described in Forest Products Laboratory Mimeograph No. 1397, determine the average specific gravity of the plies weighted according to thickness. This is done by multiplying the thickness by the specific gravity for 

each veneer, and dividing the sum of the products by the sum of the thicknesses. 

Example; What is the weighted average specific gravity of three 

veneer sheets 0.035 inch, 0,060 inch, and 0.037 inch in thickness having 

specific gravity values of 0.62, 0.40, and 0.58 respectively? 

Weighted average specific gravity = 

(0.035 x 0.62) + (0.060 x 0.40) + (0.037 x 0.58) = 0.51 

0.035 + 0.060 + 0.037 

2. From the sum of the oven-dry thicknesses of the veneer plies subtract the average oven-dry thickness of the finished plywood to obtain the 

compression resulting from manufacture. 

1

This mimeograph is one of a series of progress reports prepared by the 

Forest Products Laboratory to further the Nation's war effort, Results 

here reported are preliminary and may be revised as additional data becone available, 

Mimeo. No. 1589 -1-
3. The increase in specific gravity may be computed from the following formula: 

Gi = Gav (Tv-Tp) + 0.002 (N-1) - 0.01 

Tp 

Where: 

Gi = Increase in specific gravity of plywood over the average 

weighted specific gravity of the plies. 

Gav = Average specific gravity of plies weighted as to thickness 

(from step 1). 

Tv = Sum of thicknesses of oven-dry veneer plies, 

Tp = Thickness of oven-dry plywood. (Tv-Tp comes from step 2.) 

N = Number of plies. 

0.002 = Constant for phenolic resin film glue (0.035 gram per sq. in.). 

0.01 = Constant to correct for difference in shrinkage of plywood 

and veneer. 

The value obtained from the formula is added to the average weighted 

specific gravity of the veneer plies to give the estimated specific gravity 

of the plywood. 

Since slight variations in specific gravity and the other variables 

included in the formula do not greatly affect the value representing the 

increase in specific gravity, a value determined for a given pressing 

schedule and plywood thickness due to plywood manufacture may be added to 

the average weighted specific gravity of the veneer plies without repeating 

the computation for each panel, The actual specific gravity of the veneer 

used is of more importance in determining the specific gravity of plywood 

than the other factors. For future reference, values obtained for standard 

thicknesses and methods may be assembled in tabular form. 

Use of Chart for Estimting Increase 

in Specific Gravity 

By use of the attached chart (fig, 1) the increase in specific gravity 

of plywood over that of the average of its constituent veneer plies may be 

found graphically. The use of the chart is substituted only for the use of 

the formula, thus to obtain the estimated specific gravity of the plywood, 

the value from the chart must be added to the weighted average specific gravity of the veneer sheets as obtained according to step 1. 

Mimeo. No. 1589 -2-